There is an image forming apparatus which performs image forming using a toner. The image forming apparatus irradiates a photoreceptor drum with an optical scanning beam, to form an electrostatic latent image on the photoreceptor drum. The image forming apparatus develops the electrostatic latent image to form a toner image. For example, an image forming apparatus to form a full color image has a plurality of photoreceptor drums. The image forming apparatus irradiates on each of the photoreceptor drums with an optical scanning beam. Regarding toner images on the respective photoreceptor drums, it is necessary that they are accurately aligned so that the relative positions between the respective photoreceptor drums are not shifted. Particularly when the scanning positions of the optical scanning beams are not parallel with each other, an image quality may be deteriorated. The image forming apparatus has an adjustment mechanism of a mirror to reflect an optical scanning beam. The adjustment mechanism of the mirror supports the mirror which receives a pressing force from a pressing portion. The adjustment mechanism of the mirror has a mechanism to change a position of a projection portion to support the mirror. The adjustment mechanism of the mirror has sometimes a rotating cam and an engagement portion to fix the position of the rotating cam, as a mechanism to change the position of the projection portion. The engagement portion biases the rotating cam. The rotating cam is pressed from the mirror and the biased engagement portion. It is necessary that the rotating cam is rotated against a pressing force at the time of adjustment. Since the rotating cam receives the pressing force, the rotating cam is hard to rotate. When the rotating cam is forcedly rotated, the engagement portion or the like may be plastically deformed. When the engagement portion or the like is plastically deformed, the adjustment position of the rotating cam may go wrong.